get_post_ancestors() now returns false in addition to array()

Description

Under ​3.4, get_post_ancestors() would always return an array, in error conditions, it would return an empty array.
Under ​3.5, get_post_ancestors() returns an array of ancestors, as in 3.4, except, in the case where the $post passed in is NULL/FALSE/0/empty, then it now returns false.

This results in warning such as this: Warning: Invalid argument supplied for foreach() when the result of get_post_ancestors() was used directly in a loop,

Admitably, it's bad code which triggers this, calling get_post_ancestors() with the global $post object before it's set up in this case, but it's still a chance in behaviour which I felt needs documenting on trac. This is either a instant 'We should just return an array()' or 'wontfix: Plugins are doing it wrong and should fix their code' ticket.